I thought I already provided this answer myself.

However, this does not address the capture-groups issue I mentioned in my question: if X, Y and Z all had capture groups, obtaining their values (and knowing which regexp each comes from) would require some funky code that I need to avoid.

Let me clarify my problem: the issue is not how to build a matching regexp, but how to keep the capture groups reasonable. The reason for this is that X Y Z would be build from regexps entered by the program users, who would also provide a result pattern, possibly containing $1 $2,... So I need the correspondence between the capture groups in the input regexps and the vars to be easy to describe.